Energy, environmental and raw material supply problems daily become more critical in the production of aluminum. Recent research activity has included not only studies of a multitude of liquid extraction approaches (i.e. the so-called "acid processes") but also processes analogous to those used in the production of titanium pigment in which ilmenite or rutile is chlorined at high temperature to provide titanium tetrachloride which, after purification, can be oxidized to make high grade titanium dioxide pigment.
Although the use of aluminum chloride reduction cells may be important in the future, there still remains the problem of providing alumina for the huge existing alumina reduction facilities. Thus attention has turned to the oxidation of aluminum chloride to provide feed for the existing alumina reduction facilities.
Oxidation of aluminum trichloride, like the oxidation of titanium tetrachloride, is thermodynamically favored and has been commercially practiced with the co-oxidation of aluminum chloride in titanium tetrachloride jet oxidation reactors to make alumina coated pigments.
Unlike the oxidation of its chemically similar analogue, iron trichloride, the oxidation of aluminum chloride is not limited by thermodynamic equilibria at higher temperatures. Thus, oxidation is quite feasible in a flame reactor. However, purification of the aluminum chloride is very difficult, particularly in the separation from iron, titanium and silicon chlorides.
Aluminum chloride is a subliming solid at normal manufacturing and handling pressures and is extremely corrosive to metals, so its favorable features of oxidation over iron chloride oxidation are to some extent counterbalanced by these deficits and, more importantly, by the purity requirements for alumina to be used as reduction cell feed.
In response to environmental protection necessities, the extensive surface area of Bayer process alumina has been used to recover and recycle fluorides vaporized from the cells. As a result, the conventional specifications for alumina for reduction cell use include surface areas of the order of 75 m.sup.2 /g. But it is not essential to the aluminum reduction process that such surface areas be produced, so long as other sources of high surface area material are available for fluoride recovery purposes.
In the oxidation of aluminum cloride, the collection of aluminum chloride from the gas stream containing the chlorinator combustion gases and the impurity chlorides derived from the aluminous ore is a major problem. The complexing of aluminum chloride by alkali chlorides has been known for years, and the vapor pressure reduction by 1/1 mol or greater amount of alkali chloride is several orders of magnitude.
Even more important, the complex is in liquid form and can be easily handled compared with powder handling of aluminum chloride, a very hydroscopic and easily contaminated solid.